Titin expression as an early indication of heart and skeletal muscle differentiation in vitro. Developmental re-organisation in relation to cytoskeletal constituents.

Established myogenic cell lines of different species and tissue origin have been used to study expression and organisation of muscle-specific proteins during differentiation. Furthermore, primary cultures of rat myocard cells were used to examine these same processes during dedifferentiation. In particular, we were interested in the general mechanism that underlies the changes in the supramolecular organisation of titin during in vitro myogenesis. It became obvious that in the differentiating muscle cell cultures the redistribution of desmin, actin and myosin in a typical, differentiation state dependent fashion, always showed a certain delay when compared to titin. The sequence of changes in the assembly of cytoskeletal and sarcomeric structures observed during differentiation of the cell lines was reversed during the process of dedifferentiation in cultured rat myocard cells. These results all indicate that titin is an early marker of myogenic differentiation, both in vivo and in vitro, and the typical reorganisation of this giant molecule is independent of species or muscle cell type.